Select touching/joined vertices/edges/faces
There are several ways:

Select at least one element (face/vert/etc.) and press CtrlL to Select Linked.

Press L while hovering over an element with the mouse to select all elements connected to that element.

Select at least one element and press CtrlNumpad + to Grow the selection until everything is selected.

Select at least one face and press CtrlShiftAltF to select connected faces which have the same or similar normal/direction.


If you meant you want to select all elements that are connected to other elements, create/select a single vertex and press ShiftG&gt; Amount of connecting edges (if you want don't want to select edges that don't connect to faces, use Amount of adjacent faces instead), then press CtrlI to invert the selection.
See the manual for a full list and description of all selection tools.